As the drive power sources for portable electronic equipment such as mobile telephones (including smartphones), portable computers, PDAs, and portable music players, much use is made of alkaline secondary batteries and nonaqueous electrolyte secondary batteries, typified by nickel-hydrogen batteries and lithium ion batteries, respectively. Furthermore, alkaline secondary batteries and nonaqueous electrolyte secondary batteries are also much used as drive power sources for electric vehicles (EVs) and hybrid electric vehicles (HEVs, PHEVs), and in stationary storage battery systems in applications for curbing output variation of photovoltaic power generation and wind power generation, etc., in grid power peak load shifting applications for storing power at night and using it in the daytime, and in other applications. Particularly in EV, HEV and PHEV applications or stationary storage battery systems, high capacity and high output characteristics are required. Individual batteries accordingly get larger and are used connected in series or in parallel. Prismatic secondary batteries are widely used in such cases, because of their space efficiency.
Materials extremely rich in reactivity are used for the batteries in such applications, and particularly for nonaqueous electrolyte secondary batteries. Consequently, such batteries are required to have much higher safety than the secondary batteries used for small-sized portable equipment. Therefore, prismatic secondary batteries that are used for applications of the foregoing kinds are provided not only with a gas escape valve for releasing the battery outer casing internal pressure when it increases, but also with a current interruption mechanism for breaking the electrical connection between the external terminals and the electrode assembly inside the outer casing—as set forth, for example, in JP-A-2008-66254, JP-A-2008-66255 and JP-A-2010-212034.
For example, JP-A-2008-66254 discloses the invention of a prismatic secondary battery 50 that, as shown in FIG. 14A, includes an external terminal 53 having a through-hole 52 putting a current interruption mechanism 51 in communication with the space exterior to the prismatic secondary battery 50, and is so configured that the current interruption mechanism 51 is reliably actuated when the pressure inside the outer casing 54 increases. Furthermore, JP-A-2008-66255 discloses the invention of a prismatic secondary battery 60 that, as shown in FIG. 14B, includes an external terminal 63 having a through-hole 62 putting a current interruption mechanism 61 in communication with the space exterior to the prismatic secondary battery 60, and is so configured that the current interruption mechanism 61 is actuated when the pressure inside the outer casing 64 increases, and configured that the through-hole 62 is sealed by a membrane plug 65 of resin, in order to prevent moisture or oxygen from entering the current interruption mechanism 61 through the through-hole 62 and causing deterioration of the current interruption mechanism 61.
In the prismatic secondary batteries disclosed in JP-A-2008-66254 and JP-A-2008-66255, the through-hole is provided so that the battery exterior is in communication with the space in the current interruption mechanism that corresponds to the outside of the battery, and hence that the current interruption mechanism will be readily actuated when the pressure inside the outer casing increases. However, even if the pressure inside the outer casing increases due to some cause, the pressure of the gas that is produced in the battery interior will be extremely high during the abnormality, and there will be no simultaneous similar increase in the pressure inside the sealed space in the current interruption mechanism that corresponds to the outside of the battery. This means that there will be no substantial difference in the actuation of the current interruption mechanism, whether the space in the current interruption mechanism that corresponds to the outside of the battery is sealed or open.
JP-A-2010-212034 therefore discloses a prismatic secondary battery 70 that, as shown in FIG. 15, has a sealing body 71 that seals the mouth of the outer casing (omitted from the drawing), and a connection terminal 72 that is installed to the sealing body 71, with the object of rendering it difficult for electrolyte or cleaning fluid to enter the inside of the current interruption mechanism during manufacture. In this prismatic secondary battery 70, a current interruption mechanism 74 that interrupts the current in response to an increase in the pressure inside the outer casing is provided between the connection terminal 72 and a collector 73 that electrically connects the connection terminal 72 to the electrode assembly (omitted from the drawing); the connection terminal 72 has a through-hole 75 formed in its interior, the through-hole 75 which communicates with the space in the current interruption mechanism 74 that corresponds to the outside of the battery; and the through-hole 75 is sealed by a terminal plug 76 formed of an elastic member, so that a sealed space is formed between the through-hole 75 and the current interruption mechanism 74.
This current interruption mechanism 74 includes an inversion plate 77 that performs the function of a valve body, and the thin portion 73a of the collector 73. An annular groove 73b is formed in the thin portion 73a of the collector 73, and the inversion plate 77 is welded to the central part of the thin portion 73a. Moreover, the edge portion 77a around the periphery of the inversion plate 77 is welded to the inner circumferences of a flange portion 78a formed at the bottom end of the tubular portion of a tab member 78. The connection terminal 72 is electrically insulated from the sealing body 71 with an upper first insulating member 79 and a lower first insulating member 80 interposed therebetween, and is electrically connected to the top end of the tubular portion of the tab member 78. A second insulating member 81 of resin is disposed between the collector 73 and the inversion plate 77 at the periphery of the current interruption mechanism 74, and this second insulating member 81 is fixed to and integrated with the lower first insulating member 80 by latching-fixing portions 81a. As a result, when the pressure inside the outer casing increases, the inversion plate 77 is deformed toward the sealing body 71, and then the thin portion 73a of the collector 73 is cut through at the groove 73b. The electrical connection between the collector 73 and the inversion plate 77 is thus broken. This has the effect of stopping any further charging or discharging of the battery.
The prismatic secondary battery disclosed in JP-A-2010-212034 has high safety because it includes a current interruption mechanism. Moreover, during manufacture, the nonaqueous electrolyte or cleaning fluid, etc., will be unlikely to enter the current interruption mechanism. Thus, this invention offers the excellent advantages of a prismatic nonaqueous electrolyte secondary battery that includes high-reliability connection terminals.
However, when the battery is subjected to shock due to vibration or being dropped, for example, the electrode assembly may be shifted resulting in a shock to the current interruption mechanism 74. As a result, the connecting portion between the collector 73 and the inversion plate 77 may fracture or become cracked. Furthermore, the welded portion between the inversion plate 77 and the flange portion 78a formed at the lower end side of the tubular portion of the tab member 78 may fracture or become cracked. When the components included in the current interruption mechanism 74 are broken in this way, the conductive pathway between the collector 73 and the connecting terminal 72 may be cut off, or the current interruption mechanism 74 may cease to operate normally. For example, if a fracture or a crack exists in the welded portion between the inversion plate 77 and the flange portion 78a, gas produced in the vicinity of the electrode assembly may enter the internal space of the tubular portion of the tab 78 through the fracture or crack. Consequently, the inversion plate 77 may not be deformed toward the sealing body 71 even when the pressure in the outer casing increases. As a result, the current interruption mechanism 74 may cease to operate normally.
The present inventors conducted various verification tests on such a configuration for preventing breakage of a current interruption mechanism for a prismatic secondary battery, and then found that the problem above could be solved by forming fixing pawl portions at a plurality of locations in the second insulating member 81 to be hooked and fixed on the flange portion 78a, formed at the lower end side of the tubular portion of the tab member 78, from the outer peripheral side, so that the second insulating member 81 and the tab member 78 could be robustly joined to each other.
The inventors also found that a new problem would arise with the second insulating member 81 simply fixed on the flange portion 78a formed at the lower end side of the tubular portion of the tab member 78, with a plurality of fixing pawl portions formed at the second insulating member 81. Specifically, the second insulating member 81 is generally formed of a resin material because it should be formed of an insulating and deformable material so that the fixing pawl portions can be hooked and fixed on the flange portion 78a of the tab member 78. When each of the fixing pawl portions is formed in the second insulating member 81 of a resin material, a through-hole is formed in the main body of the second insulating member 81 at the base of the fixing pawl portion because of the production process.
If counter-electromotive current is generated in a state in which nonaqueous electrolyte is present in the through-hole formed at the base of the fixing pawl portion of the second insulating member 81 immediately after the battery internal pressure increases to bring the current interruption mechanism 74 into operation and the inversion plate 77 is thus deformed to cut off the electrical connection between the inversion plate 77 and the collector 73, sparks may occur in the through-hole, and the heat therefrom may fuse and carbonize the second insulating member 81 around the through-hole formed at the base of the fixing pawl portion of the second insulating member 81. If the carbonized portion is conductive, electrical continuity may be established again between the collector 73 and the collector tab 78 or the inversion plate 77, thereby impairing the original function.